To the Cathars, Rex Mundi was the false God - the chaotic god of material things, the Demiurge, and the embodiment of evil. The true God was one of spirituality, peace, and love, who created the heavens. Rex Mundi either created the physical world, or somehow captured it from the true God. To this line of thought, Rex Mundi also corrupted the church and worship of the true god, leading much of the church astray with physical power, wealth, and temptation in this life. As you can imagine, there was significant theological disagreement between the Cathars and the Catholics. This conflict eventually boiled into the Albigensian Crusade, wherein the Catholics wiped out the Cathars.

Rex Mundi is also frequently used as a euphemism for The Devil by some Christians (due to references in the New Testament to the "prince of this world"). (These passages of Revelations and the Gospel of John1 may be part of where the Cathars got the concept from. Far from being the apocalyptic fundamentalists of their day, however, the Cathars were more like the Western equivalent of Buddhism. They believed in reincarnation, pacifism, vegetarianism, etc. See Cathars for further details.)

Coincidentally, "King of the World" is also the title given to Cyrus the Great on the Cyrus Cylinder. The cylinder is a piece of Persian propaganda, dating back to 539 BC, which was placed sort of like a time capsule in the foundations of the temple of Marduk in Babylon. It was written in Akkadian cuneiform, not in Latin, so it doesn't actually call him "Rex Mundi" per se, but academic tradition has none-the-less bestowed the nickname Rex Mundi upon Cyrus the Great. (Additional trivia about the cylinder: In the 1970s the Shah of Iran used the cylinder as a symbol to justify the continued monarchy in Iran, and claimed it was the world's oldest Human Rights document. They cylinder currently rests at the British Museum in London.)

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Game and Story Use

The Cathari concept of Rex Mundi is an interesting one that could be used in games in several different ways, especially in settings where the Gods walk the earth and meddle in the affairs of mortals.

Does the True God still have a connection to this world? Is God dead, usurped, or merely distant? Do some religions have an easier or purer connection to their divine beings, and how might that impact the setting?

If only Rex Mundi can grant spells and blessings to priests, then that would make a very persuasive argument when those priests were trying to convert the masses. That there might be a sinister reason or source for this power would be inconsequential to many people - if they can see and feel the power, that's probably all that matters to them.

Even a more conventional Christian worldview could cast the Devil as the Gunny Hartman of a spirtual Paris Island

A Conspiracy Theorist or Prophet claims that a major Church has been usurped or infiltrated by a great evil that is merely posing as the God the people think they are worshiping. The core of the Church responds by labeling such accusations as Heresy. If left to fester this could lead to Civil War, Crusades, the Inquisition, or even Genocide.

Can the players sort out which side is telling the truth? Can they act to save the day without being branded Heretics and run out of town (or worse)?

Regardless of which side in the schism is correct about what's going on behind the scenes, there's bound to be some number of people in each faction who genuinely believe they are doing the right thing. Complicated moral puzzles like this can make for challenging role-playing, and complicated mystery plots.

Warning: Some game systems make "Detect Evil" effects really cheap and simple. Such systems will generally nip such a plot in the bud, and may kill the dramatic tension.

Depending, of course, on who is supplying them - if it's a clerical magic spell and the religion providing it has been subverted by the very evil that you're trying to detect…

The way the dualism of a usurper demiurge can be lain over an existing religion opens up some good opportunities for blending different traditions or concepts. You could add a Rex Mundi interpretation to any pantheon or religion, and draw on concepts from a variety of sources.

Conceptually, Rex Mundi is similar to the notion of the Antichrist, in that he's a powerful force in the physical world that's corrupting people and acting against the wishes of God. A GM who doesn't know much about the Cathars and Rex Mundi but still wants to use elements of them, can fill in the holes in their knowledge on Rex Mundi with ideas stolen from the New Testament and Fundamentalist's worst nightmares.

In the 70's, the Shah of Iran was propped up by the U.S.Central Intelligence Agency. Which means a clever GM could twist things around enough to interpret that as "the CIA works for The Devil", at least on a symbolic sense where the Iranian Monarchy represents Rex Mundi. A game where the Devil had infiltrated the CIA since the 70's would be pretty dark, but would put a memorable occult spin on the old government conspiracy or espionagegenre.

What if the Cathars were right? The Devil rules in Rome, and killed them all because they'd found him out. This could be used in any game set on Earth between the 11th Century and Twenty Minutes Into The Future. Just make sure you know your players first - it would be a shame to work in a major plot element that was inherently offensive to one or more of your players, and not figure it out till you'd already pissed them off.

Footnotes

1. Of course Revelations is usually attributed to the same John as the Gospel, so it would make sense for these to use the same references